Polymers such as polytetrafluoroethylene (PTFE) are known to have a low intrinsic thermal conductivity which for dense, isotropic PTFE is less than 0.35 W/mK within a wide range of temperatures from −140° C. to at least 232° C., see e.g. Price, D. M. & Jarratt, M. (2002), “Thermal conductivity of PTFE and PTFE composites”, Thermochimica Acta 392-393, p. 231-236 or Blumm, J.; Lindemann, A.; Meyer, M. & Strasser, C. (2010), “Characterization of PTFE Using Advanced Thermal Analysis Techniques”, International Journal of Thermophysics 31, 1919-1927.
It is known that the thermal conductivity of expanded porous PTFE is generally even lower due to the presence of air, typically only one tenth to about one half of the value for the dense material as a function of the porosity. These materials have therefore found application as thermal insulators (see e.g. U.S. Pat. No. 3,953,566, column 5, line 64 to column 6, line 2).
For applications as e.g. for the heat transfer from integrated circuits (“IC”) it is desired to make use of the advantageous properties of fluoropolymers, but, at the same time, thermal conductivity is required. It is known to make fluoropolymers such as PTFE thermally conductive by the incorporation of thermally conductive particles, for example metal particles, oxides or nitrides, and PCM or elastomers. Such thermally conductive PTFE composites are disclosed e.g. in U.S. Pat. No. 5,945,217 and U.S. Pat. No. 5,738,936.
However, the use of such filled fluoropolymers has several drawbacks; in particular, there is usually a loss of the outstanding properties of PTFE like stability against harsh chemical environments or a change of dielectric properties.
It is furthermore known for example from EP 2 551 324 that the intrinsic thermal conductivity of fluoropolymers such as polytetrafluoroethylene (PTFE) can be enhanced by a specific orienting, process so that sheets of the polymer are obtained which have highly increased thermal conductivity in at least one or even two directions within the plane of the sheet. However, the thermal conductivity perpendicular to the plane of the sheet remains small or even slightly decreases due to the orientation, so that the oriented sheets have a highly anisotropic thermal conductivity.
It is therefore one of the objects of the present invention to provide a process which allows to increase the thermal conductivity of thermally anisotropic sheets, such as fluoropolymer sheets, in the direction perpendicular to the sheet, so that new and improved thermally conductive articles and devices can be produced.